The present disclosure relates microphones and microphone signal processing.
Microphones include a property known as directionality. This property describes the microphone's sensitivity to sound based on the direction of a sound source relative to the front of the microphone. A microphone that is approximately equally sensitive to sound independent of direction of a sound source is known as an omni-directional or non-directional microphone. Microphones whose sensitivity to sound varies according to direction of the sound source is known as a directional microphone. Directional patterns are typically characterized by graphing the microphone's sensitivity to the sound source using a polar graph. Three common directional patterns include cardioid, hyper-cardioid, and figure-eight. A cardioid is so-called because its sensitivity pattern on a polar graph resembles a heart shape. A hyper-cardioid pattern has significantly more sensitivity to a sound source in front of the microphone than the sides or rear of the microphone. A figure-eight pattern, also referred to as a bi-directional microphone pattern, is so-called because its sensitivity pattern on a polar graph resembles the Roman numeral eight. Other types of microphone patterns are possible. These include sub-cardioid, which has nearly even front and side sound source sensitivity with a gradually diminishing sensitivity in areas behind the microphone.
There is great utility in being able to use microphones with different sensitivity patterns. For example, a microphone with a cardioid can be used advantageously for capturing live vocal performances. Because a microphone with a cardioid directional pattern is primarily sensitive to sound coming from the front and partially from the sides of the microphone, the microphone will capture primarily the vocal performance while minimizing extraneous sounds such as the drums, audience, stage-monitor speakers, or other instruments.
Most directional microphones achieve their stated directional sensitivity, known as their “polar response pattern”, over a limited portion of the audio spectrum. For example, a microphone with a cardioid polar response pattern may actually have a cardioid directional pattern only over a limited frequency range, for example, 200 Hertz (Hz) to 2 kilo Hertz (kHz). At frequencies outside of this range, the microphone may exhibit a sub-cardioid, hyper-cardioid, or even omni-directional polar pattern. This may cause undesirable leakage of sound from other undesired sound sources. For example, in an audio recording studio, a drum kit often has a separate microphone for each drum and cymbal so that the sound engineer can control the level of each drum and cymbal separately on an audio mixing board. A cardioid microphone used to pickup the sound from a snare drum only, may receive sound leakage from a nearby hi-hat cymbal in a higher frequency range where the microphone no longer exhibits a cardioid pattern. This sound leakage when mixed with the sound from the hi-hat microphone may cause an undesirable coloration of the hi-hat's sound.
In addition, directional microphones are optimized for fidelity and frequency response for sound originating from the front of the microphone capsule or “on-axis.” Directional microphones are designed to maximize rejection of sound outside of their desired pattern or alternatively, “off-axis” from the front of the microphone capsules. This characteristic can come at the expense of sound fidelity. For example, the off-axis frequency response in one or more particular directions may vary dramatically, often by more than 20 decibels (dB), which can color the sound in an undesirable and unnatural way.
Microphones can be manufactured with two or more user selectable polar response patterns. For example, a microphone may allow the user to select an omni-directional pattern, a figure-eight pattern, or a cardioid pattern. Microphone with adjustable polar patterns can exhibit worse off-axis coloration due to compromises in their design that are required to support multiple patterns in a single microphone.